Organic photovoltaic (OPV) cells are devices capable of converting solar energy into electric energy by using organic semiconductor materials, to create a photovoltaic effect. Active materials, as well as the architectures of such devices, are still evolving to meet performance and lifetime criteria enabling to widen the field of application of such technologies. The methods for manufacturing such devices also remain a constant concern.
In a conventional OPV cell structure, a substrate 1 is covered with the following successive layers:                an electrically-conductive layer 2 containing a conductive oxide used as a first electrode;        a semiconductor hole transport layer 3, also called HTL or also P layer;        an active layer 4 made of organic semiconductor material;        a semiconductor electron transport layer 5, also called ETL or also N layer; and        a conductive layer 6 used as a second electrode.        
In an inverted structure, also called NIP structure, such as schematically shown in FIG. 1, the stack has the following sequence:                substrate 1;        a conductive layer 6 containing a conductive oxide used as a first electrode;        an n (or N) semiconductor layer 5;        an active layer 4;        a p (or P) semiconductor layer 3;        a conductive layer 2 used as a second electrode or upper electrode.        
One of the main advantages of the OPV technology is the possibility to entirely form the devices by a wet process, via a varied range of printing and/or coating methods: inkjet, silk screening, slot-die, photogravure, spray coating, spin coating, flexography, or doctor blade coating . . . .
To date, the so-called “inverted” structure of OPV cells appears as the most promising since it enables to achieve the longest lifetimes. In such a configuration, the P layer, which is the hole transport layer or HTL, is thus arranged at the surface of the active layer.
Currently, the hole transport layer is generally obtained from a formulation or dispersion containing PEDOT:PSS. It comprises:                Poly(3,4-EthyleneDiOxyThiophene) or PEDOT having the following chemical structure:        
                n being a positive integer        PolyStyrene Sulfonate or PSS in proton form (right-hand side) or not (left-hand side), having the following chemical structure:        
x and y being positive integers.
PSS being a water-soluble polymer, most PEDOT:PSS formulations contain water.
Now, in OPV cells, the active layer onto which the formulation containing PEDOT:PSS is intended to be deposited is strongly hydrophobic and thus has a poor wettability by aqueous solutions.
Accordingly, the wetting ability or the ability of the PEDOT:PSS suspension to spread on the active layer is generally poor. It is thus difficult to perform continuous and uniform depositions without carrying out surface treatments.
An alternative to such treatments is to adapt the formulation intended to form the hole transport layer in order to improve its ability to wet the hydrophobic active layer.
Thus, and as an example, different solutions have been envisaged to solve this problem of compatibility between the active layer and the P layer containing PEDOT:PSS:
Document Voigt et al. (Solar Energy Materials and Solar Cells, 95, 2011, 731-734) describes the adding of a solvent (isopropanol) and/or of a non-ionic and non-conductive fluorosurfactant (Zonyl® FS300) to a PEDOT:PSS formulation to arrange it on the active layer by photogravure. However, the OPV cells thus obtained have a relatively low performance and a plasma treatment of the surface remains necessary.
Document Weickert et al. (Solar Energy Materials and Solar Cells, 95, 2010, 2371-2374) discloses the dilution of the formulation in a large quantity of alcohol (2-propanol) and the performing of a deposition by spray-coating on the active layer. However, the dry matter content being very low after dilution, the layer has a small final thickness. In relation with the specific spray-coating technique, it is possible to compensate for this small thickness by repeating the operations, but this is not feasible for most deposition techniques. Further, such a technique appears to be incompatible with the subsequent printing of an electrode, the article reporting the evaporation of the electrode.
Another option has been provided in document Lloyd et al. (Solar Energy Materials and Solar Cells, 95, 2011, 1382-1388). It comprises carrying out a plasma treatment on the active layer to make it more hydrophilic. However, such a treatment creates defects and degrades the layer.
There thus appears to be an obvious need to identify new technical solutions improving the forming of a hole transport layer containing PEDOT:PSS on the hydrophobic active layer of an optoelectronic device, and more generally of a layer containing PEDOT:PSS on a hydrophobic support.